1. Field of the Invention
This invention relates to a monochromatic-light reproduction type hologram from which recorded information can be read by an optical means, and more particularly, to a monochromatic-light reproduction type hologram that is difficult to forge using a dead copy. It also relates to a method, and an apparatus, for its image reproduction.
2. Description of the Related Art
The monochromatic-light reproduction type hologram is a sort of recording medium provided on its surface with interference fringes that form a hologram image, where monochromatic light is incident on the hologram recorded area at a given angle to cause diffraction of waves from the hologram at a given angle to thereby reproduce the hologram image. Holograms of this type have been hitherto widely used for appreciation of display or the like. In recent years, there is an increase in holograms serving as identity mediums from which recorded information can be optically read. Herein, the hologram from which recorded information can be optically read (optically information-readable hologram) refers to a medium having the hologram recorded area in which a hologram image comprised of an information pattern intended to be read afterwards has been recorded.
As the information pattern, lettering patterns of characters, numerals, ABCs, etc. are commonly used, and hologram images comprised of such lettering patterns are formed and reproduced as images on an image forming surface of a ground glass or the like to visually recognize the recorded information.
As other information patterns, relatively simple identity marks such as bar codes and calra codes are also recently used, and holograms intended for the machine reading of hologram images comprised of such identity marks, by means of an optical sensor or the like (called machine-readable holograms) are proposed (see International Patent Publication W092/04692).
These optically information-readable holograms are incorporated into cards, goods and so forth so as to be utilized as identity information for judgement on whether or not an article is genuine.
However, in conventional holograms of this type, the interference fringes that form the hologram image are set in a single directionality, and hence have had the problem that it is easy for persons having a knowledge of holograms to make a forgery using what is called a dead copy, which is to make a copy of an information pattern recorded in the hologram recorded area, on other photosensitive material by applying laser light to the recorded area of an original hologram.